This invention relates to caps for sealing the opening of screw top containers. In particular, the invention provides a screw top cap which seals bottles of carbonated liquid such as soft drinks, but is well adapted to seal other containers such as glass or PET containers with contents at above or below atmospheric pressure or having gaseous components, or requiring a hermetic seal.
Screw top caps have been used for some time to seal various containers. Although many screw tops include a separate sealing gasket within the cap, there is substantial advantage to be had in producing a one-piece cap which will effectively seal the container.
Such a one piece cap is shown in the British patent 788148 (3 Aug. 1956) which includes a continuous lip within the top portion of the cap positioned to engage against the annular end face of the opening and provide a seal between the lip and the front edge of the container with the lip curling over at its free edge. However, this cap provides a seal only against the free end edge of the container.
Australian application 15456/76 (30 Jun. 1976) discloses an alternative one-piece cap in which an annular lip extends from the inside top of the cap and engages the inner bore of a container opening so as to curl the free end of the lip in against the bore or inside surface of the container opening. However, with this cap, effective sealing requires that the inside bore of the opening be of accurate and consistent dimension. Furthermore, if aerated or other gaseous liquid is to be contained, gas pressure will tend to distort the lip and cause a seal failure.
Australian patent application 14180/83 (5 May 1983) describes a cap with two internal sealing structures. One of the structures is an annular shaped outer portion shaped to accept the outer peripheral edge of the free end of the container relying upon the pressure generated during the closing of the cap to seal against this outer edge. Further provided is an inner cylindrical lip to engage the inner bore of the container opening.
According to the present invention there is provided a closure for a container, having an externally screw threaded neck, said closure being molded in one piece from a resilient plastic material and comprising a top portion and a depending skirt which has on its internal surface a complementary screw thread, characterized in that an annular sealing rib projects downwardly from the underside of the top portion, the rib includes a first substantially cylindrical portion having a substantially cylindrical inner surface and a substantially cylindrical outer surface, the first cylindrical portion being contiguous with the top and lying adjacent to or abutting with the skirt and a second, frusto-conical, portion contiguous with the end of the first portion distal to the top and extending radially inwardly to terminate in a circular free edge, the internal diameter of the first portion being equal to or only slightly larger than the external diameter of the neck of the container to which the closure is to be attached such that, during threaded engagement of the cap with the neck, the second, frusto-conical portion will be engaged by the free end of the neck and folded back against the substantially cylindrical surface of the first substantially cylindrical portion of the rib to form a gas-tight seal between at least an outer surface of the neck of the container and the closure.
Preferably the plastic material is high density polyethylene, low density polyethylene, or polypropylene. Where the container is to be used for gaseous liquids, the plastic material must have a very low porosity to the gas. Preferably the rib is shaped and sized so that, during the threaded engagement of the closure with the container, the free edge of the rib contacts an inner surface of the top, or the surface of structure contiguous with the top, before the closure is fully engaged and such that the rib in the region proximate the free edge is pinched between the free end of the neck of the container and the top of the closure, or the structure contiguous with the top of the closure, when the closure is fully engaged with the container.
Preferably the first substantially cylindrical and second frusto-conical portions of the rib join at an included angle of at least 90xc2x0. It is also preferred that the rib is tapered, having a maximum thickness proximate the top portion of the closure and tapering to a minimum thickness at its annular free edge.
It is also preferred that the first substantially cylindrical and second frusto-conical portions of the rib smoothly join with an internal radius of from 0.1 mm to 0.5 mm, most preferably 0.2 mm. It is further preferred, that the cross-sectional thickness of the rib proximate the interface between the first and second portions is from 0.4 mm to 0.8 mm, most preferably approximately 0.6 mm.
Where the closure is adapted to seal a container with an Alcoa step finish, the first substantially cylindrical portion of the sealing rib joins the top spaced radially inwardly from the skirt so as to define a space of annular cross-section between the rib and skirt. Where the container neck has a standard finish the rib is closely spaced from, or contiguous with, the skirt.